Pressure sensor

ABSTRACT

A pressure sensor for sensing a pressure level of a medium. The pressure sensor includes a rigid substrate having a medium contacting side and a pressure sensitive resistor mounted on the medium contacting side. The resistor exhibits a change in resistance in response to pressure changes on the resistor above a predetermined threshold. Other embodiments of the invention are shown using a wheatstone bridge with pressure sensitive resistors and resistors that are insensitive to pressure changes.

CROSS-REFERENCE TO CO-PENDING AND RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/716,752, filed Nov. 19, 2003 which claims priority from U.S.Provisional Patent Application Ser. No. 60/490,648, filed Jul. 28, 2003,the contents of which are herein incorporated by reference in entirety.

BACKGROUND

The present invention relates to pressure sensors in general and inparticular to a pressure sensor that eliminates the use of a diaphragmbetween the sensor and pressure to be measured.

Conventional devices for high pressure measurement in severeenvironments rely on a diaphragm in conjunction with a pressure sensingelement. Various pressure sensing elements have been used such as straingages, piezoresistive devices and semiconductor based sensing elements.These devices are constructed such that the diaphragm is positionedbetween the pressurized process media and the pressure sensing element.The diaphragms are subject to mechanical fatigue and therefore limit theservice life of conventional high pressure sensors. A diaphragm freehigh pressure sensor as presented herein is therefore desirable.

SUMMARY OF THE INVENTION

It is a feature of the present invention to provide a pressure sensorfor sensing pressure of a media and providing an electrical signal thatis indicative of the pressure level.

It is a feature of the present invention to provide a method formeasuring pressure of a pressurized medium above a predeterminedthreshold pressure. The method uses a rigid substrate bearing a filmresistor. The film resistor is exposed to the pressurized medium. Theelectrical resistance of the film resistor is detected.

It is a feature of the present invention to provide a high pressuresensor for detecting the pressure of a pressurized medium above apredetermined threshold pressure. The sensor includes a rigid substratehaving a medium contacting side and an applied film resistor mounted onthe medium contacting side. The resistor exhibits a change in resistancein response to pressure changes on the resistor above a predeterminedthreshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a resistor in a pressurized medium.

FIG. 2 is a graph of resistance versus pressure for the resistor of FIG.1.

FIG. 3 is a cross-sectional view of a pressure sensor.

FIG. 4 is an enlarged cross-sectional view of the strain resistors andsupporting substrate of FIG. 3.

FIG. 5 is a top view of the strain resistors and supporting substrate ofFIG. 4.

FIG. 6 is a bottom view of the strain resistors and supporting substrateof FIG. 4.

FIG. 7 is a graph of resistance versus pressure data for the pressuresensor of FIG. 3.

FIG. 8 is a cross-sectional view of another embodiment of a pressuresensor.

FIG. 9 is a cross-sectional view of the sensor substrate of FIG. 8.

FIG. 10 is a top view of the substrate of FIG. 9.

FIG. 11 is a bottom view of the substrate of FIG. 9.

FIG. 12 is a cross-sectional view of an alternative sensor substrate.

FIG. 13 is a cross-sectional view of another alternative sensorsubstrate.

FIG. 14 is a cross-sectional view of an alternative resistor design in apressurized medium.

It is noted that the drawings of the invention are not to scale. In thedrawings, like numbering represents like elements among the drawings.

DETAILED DESCRIPTION First Embodiment

Referring to FIG. 1, a pressure sensor 23 is shown inside a pressurevessel 22. Pressure vessel 22 contains a pressurized medium 20.Pressurized medium 20 can be any fluid medium that can be pressurized orcompressed. For example, any liquid or gas. The arrows indicate that thepressurized medium is acting with equal or isostatic pressure againstall surfaces of the pressure vessel 22. Pressure sensor 23 includes arigid substrate 24 having a medium contacting surface 26 and a backsurface 28 that may or may not be exposed to the pressurized medium.Surface 26 is at least partially exposed to the pressurized medium 20. Apressure sensitive resistor 30 is located on surface 26. Resistor 30 hasa surface area 32 that is directly exposed and in contact withpressurized medium 20. The exposed surface area 32 and substrate areisostatically compressed by medium 20.

The resistor 30 can be an applied film resistor. Preferably resistor 30is a thick film resistor that is screen printed onto a ceramic substrate22 and fired in an oven. A preferred resistor composition is Heraeus8241 resistor material which is commercially available from HeraeusCorporation of West Conshohocken, Pa. Other types of resistors can alsobe used such as thin film, discrete or diffused silicon resistors.

FIG. 2 shows a graph of resistance versus pressure for pressure sensor23. The resistor changes resistance in response to the applied pressurelevel. The resistance across resistor 30 is about 410 ohms when medium20 is pressurized to 5000 pounds per square inch. The resistance acrossresistor 30 is about 360 ohms when medium 20 is pressurized to 50,000pounds per square inch. The resistance value is linear with pressure.The pressure was cycled between 5000 and 50,000 pound per square inch inorder to test the hysteresis and repeatability of the pressure sensor.The sensor exhibited less than 0.2 percent error as it was cycled whichis excellent for a pressure sensor. Resistor 30 has dimensions of xlength, y width and z thickness. A resistor 30 that was fabricated andtested had dimensions of 0.014 inches in length, 0.014 inches in widthand 10 microns in thickness. It is noted that substrate 24 is rigid andthick enough that it does not bend under pressure. In other words,substrate 24 does not act like a diaphragm.

Pressure sensor 23 is most useful for measuring large changes inpressure and for use with high pressures. This is due to the fact thatthe resistance change with pressure is small over a large pressurerange. Pressure sensor 23 is best used with pressure ranges above 500pounds per square inch. Pressure sensor 23 can be used to detectpressures down to 0 pounds per square inch (gauge pressure). In use, therigid substrate 24 with a film resistor 30 is exposed to the pressurizedmedium 20 above a pre-determined threshold pressure. A voltage isapplied across resistor 30 and the voltage drop across the resistor isconverted to a resistance value using ohm's law. In this manner, theelectrical resistance of the film resistor is detected. The pressurelevel of the medium is proportional to the resistance value.

Preferred Embodiment

Turning now to FIGS. 3-6, a preferred embodiment of a pressure sensor 40is shown. Pressure sensor 40 has a cylindrical housing 42 that has ends46 and 47. A bore 45 extends through the center of housing 42. At end46, the housing has a cavity 48 that extends into the bore 45. A step 51is located at the bottom of cavity 48. The housing has an outer surfacewith male threads 43. The housing can be made out of metal such asstainless steel. Threads 43 are used to attach the pressure sensor topressure vessel 22. Several flat surfaces 44 are placed on the outsideof housing 42 so that a wrench can rotate the sensor. Male threads 43would mate with female threads (not shown) on the pressure vessel. Othermethods of attaching the housing could be used such as press fitting.Housing 42 has a groove 49 located around the circumference of cavity48. O-ring 50 fits into groove 49. O-ring 50 makes a seal betweenhousing 42 and substrate 60.

A substrate 60 is mounted in cavity 48. Substrate 60 has a mediumcontacting or pressurized surface 61 and a non-pressurized surface 62.Substrate 60 can be an alumina ceramic, low temperature co-firedceramic, glass or a metal with an applied dielectric surface. A pair ofpressurized resistors 64 are located on surface 61 and a pair ofnon-pressurized resistors 66 are located on surface 62. Resistors 64 areexposed to the pressurized medium. Resistors 66 are not exposed to thepressurized medium. Resistors 64 and 66 can be conventional thick filmresistors that are manufactured using conventional thick film processingtechniques. A preferred resistor composition is Heraeus 8241 resistormaterial which is commercially available from Heraeus Corporation ofWest Conshohocken, Pa. Resistors 64 and 66 can also be thin filmresistors, plated resistors or resistors that are diffused into asilicon substrate. Conductors 68 are located at each end of resistors 64and 66. Several conductive vias 70 extend through substrate 60 andelectrically connect with conductors 68. Metal pins could also be usedin place of the vias.

Resistors 64 and 66 are connected to form a Wheatstone bridge. In theWheatstone bridge, resistors 64 are called the sense resistors andresistors 66 are called the reference resistors. Resistors 64 changeresistance in response to pressure changes. Resistors 66 have arelatively constant value as they are not exposed to changes inpressure. Since the resistors 64 and 66 are thermally coupled bysubstrate 60, they operate at about the same temperature. This minimizesthe amount of temperature compensation that is needed and allows formore accurate pressure readings.

Four wires 72 are connected to conductors 68 on surface 62. Wires 72supply a voltage and ground potential to the resistors. The wires can beconnected to the conductors by soldering or by welding. Conductors 68could be connected to terminals or to a connector. Wires 72 would beconnected with a power source and to conventional signal processingcircuitry for calibration and temperature compensation.

FIG. 7 shows a graph of pressure versus output voltage withoutamplification for pressure sensor 40. The wheatstone bridge changesoutput voltage in response to the applied pressure level. The wheatstonebridge drive voltage is 5 volts and the bridge output was notcompensated for temperature changes. The voltage is about 0 milli-voltswhen medium 20 is pressurized to 0 pounds per square inch. The voltageis about 200 milli-volts when medium 20 is pressurized to 25,000 poundsper square inch. The voltage is linear with pressure. For this exampleresistors 64 and 66 each had dimensions of 0.014 inches in length, 0.014inches in width and 10 microns in thickness. The resistors were notlaser trimmed. It is noted that substrate 60 is rigid and does not bendunder pressure. In other words, substrate 24 does not act like adiaphragm. The pressure was repeatably cycled between 0 and 25,000pounds per square inch in order to test the hysteresis and repeatabilityof the pressure sensor. The sensor exhibited less than 0.2 percent erroras it was cycled which is excellent for a pressure sensor.

Third Embodiment

Referring to FIGS. 8-11, another embodiment of a pressure sensor 80 isshown. Pressure sensor 80 is similar to sensor 40. In sensor 80,substrate 60 has been replaced by a substrate 82.

Substrate 82 has a medium contacting side 61, also referred to as apressurized surface 61, and a non-pressurized surface 62. Ceramicsubstrate 82 can be an alumina ceramic or can be a low temperatureco-fired ceramic. A pair of pressure sensitive resistors 84 are locatedon surface 61 and a pair of non-pressure sensitive resistors 86 arelocated on surface 61. Both resistors 84 and 86 are exposed to thepressurized medium. Resistors 84 and 86 can be conventional thick filmresistors that are manufactured using conventional thick film processingtechniques. Resistor 84 is made of Heraeus 8241 resistor material whichis commercially available from Heraeus Corporation of West Conshohocken,Pa. Resistor 86 is made from Pyramide resistor material from CTSCorporation of Elkhart, Ind. Pyramide resistor material is made from amixture of Ruthenium Oxide and Glass. It has a small particle size anddoes not change resistance under changes in pressure. Resistors 64 and66 can also be thin film resistors, plated resistors or resistorsdiffused into a silicon substrate. Conductors 68 are located at each endof resistors 84 and 86. Several conductive vias 70 extend throughsubstrate 82 and electrically connect with conductors 68. Metal pinscould be used in place of vias 70.

Resistors 84 and 86 are connected to form a Wheatstone bridge. In theWheatstone bridge, resistors 84 are called the sense resistors andresistors 86 are called the reference resistors. Resistors 84 changeresistance in response to pressure changes. Resistors 86 have arelatively constant value as they are not responsive to changes inpressure. Locating the resistors 84 and 86 close to each other on thesame substrate allows them to operate at about the same temperature.This minimizes the amount of temperature compensation that is needed andallows for more accurate pressure readings.

Four wires 72 are connected to conductors 68 on surface 62. Wires 72supply a voltage and ground potential to the resistors. The wires can beconnected to the conductors by soldering or by welding. Conductors 68could be connected to terminals or to a connector. Wires 72 would beconnected with a power source and to conventional signal processingcircuitry for calibration and temperature compensation. The pressureversus output voltage for pressure sensor 80 would be similar to thatfor pressure sensor 40.

Fourth Embodiment

Turning now to FIG. 12, a cross-sectional view of an alternative sensorsubstrate 87 is shown. Substrate 87 is similar to substrate 60 exceptthat a covercoat 88 has been placed over resistors 64 and 66. Covercoat88 can be a screened and fired glass covercoat or can be an organiccovercoat such as preflorinated polyether. The covercoat protects theresistors from corrosive environments while at the same time allowingthe resistors to be compressed by the applied pressure of the medium.

Fifth Embodiment

Referring to FIG. 13, a cross-sectional view of an alternative sensorsubstrate 90 is shown. Substrate 90 is similar to substrate 60 exceptthat the ceramic substrate 90 is now comprised of two layers 91 and 92of a low temperature co-fired ceramic material (LTCC). LTCC materialsare commercially available from Dupont Corporation of Wilmington, Del.Resistor 64 has been buried within the ceramic layers. The buriedresistor is protected from corrosive environments while at the same timeallowing the resistors to be compressed by applied pressure.

Sixth Embodiment

Referring to FIG. 14, a cross-sectional view of an alternative resistordesign in a pressurized medium is shown. In FIG. 14, a pair of resistors94 are shown exposed to medium 20. Resistors 94 have a length, a widthand a height that define a resistor volume. The resistor 94 has aresistance that varies with applied pressure. The resistor is in directcontact with the pressurized medium and is uniformly compressed by thepressurized medium such that the resistor volume changes with a changein pressure. The change in resistor volume generates a change in theresistance of the resistor. A pair of terminals 95 are attached toopposing sides of resistor 94. The terminals provide an electricalconnection from the resistor to an external electrical circuit (notshown).

The resistors of FIG. 14 would preferably be connected in a wheatstonebridge configuration with two of the resistors being exposed to thepressurized medium and two of the resistors being out of the pressurizedmedium.

One of ordinary skill in the art of designing and using pressure sensorswill realize many advantages from using the present invention. Theelimination of the diaphragm of prior art sensors eliminates one of themajor sources of sensor error and failure and also results in a lowercost assembly.

An additional advantage of the present invention is improved accuracy.Since the pressure sensitive resistors are in direct contact with thepressure vessel, the sensor can react directly to changes in pressure.Sensors of the prior art have a diaphragm located between the sensor andthe pressure vessel. The diaphragm reduces response time and accuracy ofthe sensor.

While the invention has been taught with specific reference to theseembodiments, someone skilled in the art will recognize that changes canbe made in form and detail without departing from the spirit and thescope of the invention. The described embodiments are to be consideredin all respects only as illustrative and not restrictive. The scope ofthe invention is, therefore, indicated by the appended claims ratherthan by the foregoing description. All changes which come within themeaning and range of equivalency of the claims are to be embraced withintheir scope.

1. A method for measuring pressure of a pressurized medium above apredetermined threshold pressure, the method comprising: providing arigid substrate bearing a pressure sensitive resistor; exposing thepressure sensitive resistor directly to the pressurized medium; anddetecting the electrical resistance of the pressure sensitive resistor.2. The method for measuring pressure according to claim 1, wherein themeasured pressure range is greater than 500 pounds per square inch. 3.The method for measuring pressure according to claim 1, wherein thepredetermined threshold pressure is between 0 pounds per square inch and50,000 pounds per square inch.
 4. The method for measuring pressureaccording to claim 1, wherein the substrate is ceramic and the resistoris a thick film resistor.
 5. The method for measuring pressure accordingto claim 1, wherein a conductor is attached to each end of the resistor.6. The method for measuring pressure according to claim 1, wherein thesensor does not have a diaphragm.
 7. The method for measuring pressureaccording to claim 1, wherein the pressurized medium presses uniformlyon all exposed surfaces of the resistor.
 8. The method for measuringpressure according to claim 3, wherein the substrate has a plurality ofvias extending therethrough, the vias connected to the conductors. 9.The method for measuring pressure according to claim 1, wherein avoltage is applied to the resistor, the pressure sensor being adapted tooutput an electrical signal that changes in response to a pressurechange in the media.
 10. The method for measuring pressure according toclaim 1, wherein four resistors are located on the substrate and areconnected to form a wheatstone bridge.
 11. The method for measuringpressure according to claim 1, wherein the substrate has a first surfaceand a second surface, two resistors being mounted to the first surfaceand two resistors being mounted to the second surface, the resistorsconnected together to form a wheatstone bridge.
 12. A pressure sensorfor sensing a pressure level of a pressurized medium, comprising: a) asubstrate having a first surface and a second surface, the first surfacecommunicated with the medium; and b) at least one resistor mounted tothe first surface, the resistor having a surface area, the resistorbeing directly exposed to the pressurized medium such that thepressurized medium isostatically compresses the resistor, the resistorbeing adapted to change resistance in response to the pressure level.13. The pressure sensor according to claim 12, wherein the sensedpressure range is above 500 pounds per square inch.
 14. The pressuresensor according to claim 12, wherein the resistor is an applied filmresistor.
 15. A high pressure sensor for detecting the pressure of apressurized medium above a predetermined threshold pressure, the sensorcomprising: a) a rigid substrate having a medium contacting side; and b)an applied film resistor mounted on the medium contacting side, theresistor exhibiting a change in resistance in response to pressurechanges on the resistor above a predetermined threshold.
 16. Thepressure sensor according to claim 15, wherein the sensor does not havea diaphragm.
 17. The pressure sensor according to claim 15, wherein aconductor is attached to each end of the resistor.
 18. The pressuresensor according to claim 15, wherein the substrate does not bend underpressure.
 19. The pressure sensor according to claim 15, wherein thesubstrate is mounted to a housing.
 20. The pressure sensor according toclaim 19, wherein the housing is secured to a pressure vessel, thepressurized medium contained within the pressure vessel.
 21. Thepressure sensor according to claim 15, wherein the resistor is coveredby a covercoat.
 22. The pressure sensor according to claim 15, whereinthe resistor is buried within the substrate.
 23. A pressure sensor foruse with a pressurized medium, comprising: a) at least one resistorhaving a length, a width and a height, the length width and heightdefining a resistor volume, the resistor having a resistance that varieswith applied pressure, the resistor adapted to be in direct contact withthe pressurized medium, the resistor adapted to be uniformly compressedby the pressurized medium such that the resistor volume changes with achange in pressure, the change in resistor volume generating a change inthe resistance of the resistor; and b) a pair of terminals attached toopposing sides of the resistor, the terminals providing an electricalconnection between the resistor and an external electrical circuit. 24.The pressure sensor according to claim 23, comprising four resistorsconnected in a wheatstone bridge, two of the resistors being exposed tothe pressurized medium and two of the resistors being out of thepressurized medium.